The present invention relates to a high voltage DC generator for use in the supply of high DC voltages to a cathode ray tube.
One of the important factors that determines the picture quality of a television receiver rests on the quality of the high voltage DC generator that supplies high DC voltages to the cathode ray tube. To improve the quality of the DC generator it is necessary to minimize output voltage fluctuation and ringing.
The high voltage supplies to a cathode ray tube are usually obtained from a horizontal deflection circuit by utilizing the effect of high frequency switching action of the deflection circuit on the inductance of a flyback transformer. However, the use of a flyback transformer having as many turns of wire as desired obviously results in a bulky transformer and an increase in leakage inductance between the primary and secondary windings and an increase in stray capacitance in the secondary. Since the increases in leakage inductance and stray capacitance reduce the frequency of a damped oscillation known as ringing and make it difficult to tune the ringing frequency to a desired odd integral multiple of the fundamental frequency of the flyback pulse, whereby the ringing component can be reduced to an acceptable level.
It is also desirable to allow transformers of the same type to be used in applications in which the line frequency of the horizontal deflection system may differ from one system to another or in which the scan size is controlled according to particular purposes. In these applications, the duration of the flyback pulse must be controlled to fit to the different requirements and the ringing frequency adjusted to maintain proper frequency relationship with the frequency of flyback pulse. Otherwise the ringing frequency would deviate from the optimum point, and there results an increase both in ringing and voltage output fluctuation.
A high voltage DC generator, shown and described in copending U.S. patent application Ser. No. 591,748 filed Mar. 21, 1984 and assigned to the same assignee as the present invention, comprises a horizontal deflection circuit, a flyback transformer and a rectifier which includes two pairs of diodes each connected in series from one of the opposite end terminals of the secondary winding of a flyback transformer. Two capacitors are cross-coupled from the junctions of the diodes of each pair to the end terminals of the secondary winding of the flyback transformer.
The deflection circuit includes a switching transistor which causes a current to flow through the primary winding of the flyback transformer in response to a line frequency input and induces a voltage pulse in the secondary winding of the transformer in the retrace period of each line scan. The voltage developed in the secondary winding biases one of the diodes of each pair into conduction to charge the capacitors in one direction. In the trace period that follows, the other diodes of each pair are rendered conductive to charge the capacitors in a direction that is opposite to the direction in which these capacitors are charged during the retrace period. A DC output voltage which is twice the voltage induced in the secondary winding of the flyback transformer is obtained across the output terminals.
The voltage developed in the transformer secondary winding during the retrace period of a line scan entails a ringing voltage in the trace period of the line scan. This ringing voltage is a damped oscillation at an odd harmonic of the line frequency and is determined by a leakage inductance between the primary and secondary windings of the flyback transformer and a stray capacitance produced by the final anode of the cathode ray tube. This ringing component impairs the picture quality.
Another factor that determines the picture quality is the range in which the DC output of the generator tends to fluctuate with a varying beam current of the cathode ray tube. To minimize the output voltage fluctuation a bleeder resistor is connected to the output terminals of the generator. It is found, however, that the bleeder resistor affects the ringing component in such a conflicting manner that the ringing increases with the increase in bleeder resistance. Specifically, with a bleeder resistor of 100 megohms, the output voltage fluctuation is acceptable, but the ringing is not acceptable. A bleeder resistor of 50 megohms, on the other hand, reduces the ringing component to an acceptable value but increases the output voltage fluctuation and bleeder power loss to inacceptable values.